1. Field of the Invention
The present invention relates to a method and a device for testing the integrity of filter elements, in particular filter elements for sterile filtration.
2. Description of the Related Art
Filtration systems for sterile filtration of gases and liquids are employed in many areas of technology, in particular in the pharmaceutical industry, food production, the electronics industry etc. Sterile filters with membranes made of different polymers are employed for sterile filtration. In the sterile filtration of gases and in the sterile ventilation from vessels, in particular in the pharmaceutical industry, standard sterile filters with hydrophobic membranes such as, for example, membranes based on polytetrafluoroethylene (abbreviated to PTFE) are employed.
Conventional methods of testing hydrophobic filter elements are either wetted with alcohol and measured at diffusion/bubble point in the single-digit minute range with the known disadvantages of subsequent drying. In a second test method water pressure is applied to the hydrophobic filters and testing for water flow or water intrusion is performed in the double-digit minute range. Only filter surfaces greater than or equal to 900 cm2 can be measured with the WFT and WFI. Smaller filter surfaces must be wetted with alcohol with the known disadvantages.
Previous testing technologies in the single-digit minute range for sterile filters are based on measurement of a pressure rise on an outlet side or measurement of a pressure drop on an inlet side of a pleated PTFE filter element wetted with an isopropanol/water mixture. With a given volume, a diffusion through the filter element can be calculated from the measured pressure rise or pressure drop. This testing technology is regarded as an indirect testing method and has the disadvantages that the testing method is temperature-dependent and requires a reference volume for carrying out the measurement. Furthermore, the known method has the disadvantage that the filter element must be wetted with alcohol for the measurement. As a result, a final drying process in explosion-proof drying cabinets is necessary, which can last for approximately 8 hours.
A testing method which omits the use of alcohol is the so-called water flow test (abbreviated to WFT). In this case wetting of the filter element with alcohol can be omitted if RO water is used as the testing medium. RO water is completely desalinated water. Because of the hydrophobicity of PTFE, the filter element is largely dry after the completion of the testing process, so that the subsequent drying process can be significantly shortened.
In the WFT the PTFE filter element is positioned in a vessel which has been completely filled at the inlet end with RO water. During the test process a test pressure is built up by means of the RO water below a filter-specific intrusion pressure of water which leads to a pressure drop in a previously determined reference volume. The quantity of RO water which has been pressed through the membrane of the PTFE filter element in a predetermined time interval is calculated from the measured pressure drop. In order to produce a measurable pressure drop, testing times in the double-digit minute range of usually 23 minutes per testpiece are necessary, which is why the WFT method is not suitable for an industrial scale and is not suitable for testing very small filter elements. The WFT method is temperature-dependent and determines the integrity of the filter element to be tested only indirectly by means of a previous net volume measurement.
The object of the invention is to enable improved integrity testing of filter elements. In particular, the disadvantages of the WFT method such as, for example, the long measurement time, the limited accuracy of measurement and/or the restricted field of application should be reduced.